I'm on scenario 16 and almost every scenario has been easy, boring filler. Move forward, slap around some grunts, then kick an easy bosses ass. I dunno what they're doing with the plot but the gameplay isn't up to par. I'm hoping the game heats up soon because 16 scenarios of grunt filler is not cool. The animations are amazing but can't carry the game on their own.
On the ability slot system, I've kept it simple and stuck with 3 of the same on one pilot or robot. I haven't bothered trying to mix and match as it doesn't seem like there's much benefit so far, and as you noted it can get complicated. I'm not too hot on the system.. overall it's pretty ignorable so far.
I haven't managed to pull off a "maximum break", partly because the scenarios so far end too quickly, partly because I haven't had the need to set one up yet. Fleeing bosses retreat at very low (7-8k) HP values so it's rarely a struggle to do enough damage.